The present invention relates in general to the problem of detecting pinking (knocking) in internal combustion engines, particularly as far as controlled-ignition (Otto) engines for vehicular use are concerned.
Pinking occurs in such engines for various reasons (supercharging, high compression ratio, certain loading conditions, etc.) due to self-ignition of the explosive mixture when the piston is near top dead centre (TDC). The pinking is noticeable from the outside as a characteristic metallic sound known as "pinging".
In the event of pinking, there is no great difference in the form of the pressure cycle (between the compression, combustion and power stages of the engine) but oscillations arise which are superimposed on the normal cycle in correspondence with its maximum.
The frequency of these oscillations is determined by the dimensions of the combustion chamber when pinking occurs. The resonant frequencies f.sub.nm can be calculated theoretically by considering a system with a cylindrical cavity with a diameter equal to the bore: ##EQU1## where beta.sub.nm are the values of a Bessel function; C is the speed of sound: C=20T.sup.1/2 (T=the temperature of the fluid); D is the diameter of the cylinder (the bore). For example, for a bore with a value of approximately 80 mm, it is possible to calculate that the principal modes of vibration correspond to frequencies of 8, 13.8, 15.8 and 18.2 KHz.
The calculations is valid for the situation which occurs up to 5.degree.-10.degree. after TDC; subsequently, the resonant frequencies shift to lower frequencies as a result of the decrease in the temperature of the fluid in the cylinder (delta f.congruent.2KHz). This shift also takes place in relation to the rate of rotation (r.p.m.) of the engine, since the temperature of the fluid increases as the rate of rotation increases.
The pressure oscillations which are generated in the presence of pinking are transmitted through the walls of the cylinder and can be detected by a sensor. However, this sensor also detects the vibrations due to normal combustion and those due to the moving mechanical parts. The output signal of the sensor must therefore be processed to separate the signal due to the pinking from the background noise which varies in dependence on the rate of rotation (r.p.m.) of the engine and the extent of wear.
More specifically, pinking can be detected by means of a pressure sensor facing into the combustion chamber, or by means of a piezoelectric accelerometer mounted on the engine block. The latter is usually preferred for reasons of cost and reliability, even though the pressure sensor identifies pinking directly.
The piezoelectric accelerometer is characterized by a frequency response with a lower zone of use in which the output is proportional only to the amplitude of the variations, and a higher zone in which the response also depends on the frequency of the vibrations.
The accelerometric sensor is mounted on the engine block, typically on the head or near the intake manifold: since the intensity of the vibrations depends on its position on the engine, it would seem that the sensor should be located in a certain position. In fact, there is no appreciable improvement in the signal/noise (S/N) ratio because the intensity of the background noise (vibrations not due to pinking) is also greater.
A spectral analysis of the signal in the absence of pinking shows that the noise level decreases beyond a frequency of approximately 6KHz. This suggests that a better signal/noise ratio may be obtained in correspondence with the resonant frequencies of the higher modes; in fact, in the presence of pinking, the levels of the signals at 8 and 13.8 KHz show no significant differences, unlike the noise which decreases.
In the presence of pinking, the spectrum of the signal gives three types of trace: the presence of peaks at both 8 and 13.8 KHz; the presence of only one peak at 8 KHz and the presence of only one peak at 13.8 KHz. This probably depends on the intensity of the phenomenon: the higher the compression ratio, the more of a peak there is at 13.8 KHz.